1. Field of the Invention
The present invention relates to a silicon-containing soluble polyimide precursor, a cured material of the polyimide which is obtained by heating the above-mentioned precursor to crosslink/cure it, and a method for preparing them.
2. Description of the Prior Art
Polyimide resins have maximum heat resistance and low coefficient of thermal expansion considering organic polymer compounds, but these characteristics are inferior to those of inorganic compounds.
In order to improve surface hardness, wear resistance and the like of the polyimide resin with the intention of causing these properties to come up to those of inorganic compounds, a means for filling the polyimide resin with an inorganic material is taken. In this case, however, breakage is liable to occur between the organic material and the polyimide resin inconveniently, and therefore such a means is not always considered to be preferable.
On the other hand, silica which is one example of the inorganic materials has practically preferable characteristics, e.g., low coefficient of thermal expansion and high hardness in addition to excellent heat resistance, but it is brittle and poor in processability and moldability. For this reason, use applications of silica are limited. For the purpose of improving the processability and moldability, a variety of compounds have been synthesized in which groups which are bonded to each silicon are partially replaced with alkyl groups. This technique is successful to some extent, for example, as polydimethylsiloxanes and the like, but such compounds involve drawbacks such as noticeable deterioration of heat resistance, perceptible increase in coefficient of thermal expansion and outstanding degradation of hardness.
For the elimination of these drawbacks, many techniques of chemically combining polyimides with silicon compounds have been reported (e.g., Japanese Patent Laid-Open Publication Nos. 143328/1982, 7473/1983 and 13631/1983). These techniques are based on the partial replacement of a diamine component, which is the raw material of the polyimide, by a polydisiloxane which is terminated with diamines at both the ends thereof.
Japanese Patent Publication No. 32162/1983 has suggested a crosslinked polyimide having a siloxane group which can be prepared by mixing a polyamide acid, which is terminated with reactive silicon compounds at both the ends thereof, with a polysiloxane having hydroxyl groups at both the ends thereof, and then heating the mixture.
Furthermore, as a process of forming silica films, there has been suggested a technique of calcining a reactive silane such as an alkoxy silane or an acetoxy silane (e.g., Japanese Patent Publication Nos. 16488/1977 and 20825/1977, Japanese Patent Laid-Open Publication Nos. 34258/1980 and 250032/1986, and U.S. Pat. No. 4,408,009).
Products disclosed in the above-mentioned Japanese Patent Laid-Open Publication Nos. 143328/1982, 7473/1983 and 13631/1983 still have disadvantages such as noticeable deterioration of heat resistance, perceptible increase in coefficient of thermal expansion and outstanding degradation of hardness, as in the above-mentioned polydimethylsiloxane and the like.
The compound disclosed in Japanese Patent Publication No. 32162/1983 is excellent in affinity for inorganic compounds but cannot provide any materials having low coefficients of thermal expansion.
With regard to the above-mentioned technique of calcining a reactive silane such as an alkoxy silane or an acetoxy silane, films synthesized by this technique are very brittle, and the thickness of the films is at most several thousand angstroms.
As discussed above, the conventional techniques have various problems, and thus it is now demanded to develop satisfactory substances for the boundary between inorganic and organic materials.